REGULATION OF TEMPERATURE 



313 



season. This fact, together with the saving in transportation, 

 makes it possible to grow greenhouse crops at a profit, even though 

 the cost of glass houses, fuel, and hand labor is great. 



In nearly every large city of the North and in many smaller 

 towns there are extensive greenhouse plants devoted to the grow- 

 ing of vegetables, some individual growers having as much as ten 

 acres under glass. Lettuce is grown in this manner more exten- 

 sively than any other crop (Fig. 189), though it is quite a common 

 practice to grow spring crops of cucumbers or tomatoes in houses 

 devoted to lettuce during the coldest months. 



Fn;. 1S8. — Hotbed of head lettuce approaching maturity. 



Hothouse vegetables are not marketed exclusively in the cities 

 where they are grown. Considerable shipping trade has been 

 developed. In this case there is some competition with southern 

 products, but the superior quality of the forced product quite 

 largely eliminates this factor. 



Regulation of Temperature. — The temperature and moisture 

 requirements of a given vegetable are essentially the same whether 

 it is grown under glass or in the open; but in growing a crop under 

 glass, especially in a modern greenhouse where plenty of heating 

 pipes have been installed and the arrangement of valves makes it 



